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Advanced Settings
the bridge priority is a configurable parameter. The smaller the bridge ID is, the higher the
bridge priority is. The root bridge is the bridge with the lowest bridge ID.
2
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Root Bridge
There is only one root bridge in the networking and it is changeable as the network
topology changes. Initially, all devices regard themselves as the root bridge and generate
their own configuration BPDUs and send them out periodically. When the network
topology becomes stable, only the root bridge device will send configuration BPDUs out
and other devices will forward these BPDUs.
3
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Root Port
The root bridge port is the port that has the lowest path cost from this bridge to the Root
Bridge and forwards packets to the root. There is only one root port on the non-root bridge
device and no root port on the root bridge devices.
4
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Designated Bridge and Designated Port
Designated bridge: As for a device, it is the device that connects to and forwards BPDUs
the host. As for a LAN, it is the device that forwards BPDUs to the network segment.
Designated port: As for a device, it is the port that forwards BPDUs to the host. As for a
LAN, it is the port that forwards BPDUs to the network segment.
5
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Path Cost
The parameter for choosing the link path by STP. By calculating the path cost, STP
chooses the better links and blocks the redundant links so as to disbranch the
ring-network to form a tree-topological ring-free network.
BPDU Priority in STP mode
Assuming two BPDUs: BPDU X and BPDU Y
If the root bridge ID of X is smaller than that of Y, X is superior to Y.
If the root bridge ID of X equals that of Y, but the root path cost of X is smaller than that of
Y, X is superior to Y.
If the root bridge ID and the root path cost of X equal those of Y, but the designated bridge
ID of X is smaller than that of Y, X is superior to Y.
If the root bridge ID, the root path cost and designated bridge ID of X equal those of Y, but
the designated port ID of X is smaller than that of Y, X is superior to Y.
STP Computing Process
Initial Status
Initially, each switch regards itself as the root, and generates a configuration BPDU for
each port on it as a root, with the root path cost being 0, the ID of the designated bridge
being that of the switch, and the designated port being itself.
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